1. Technical Field
The present invention relates generally to apparatus for detecting extraneous substances on an glass plates, and more particularly to an apparatus for detecting extraneous substances on a glass plate so that when extraneous substances are sticking to the front surface and the back surface of the glass plate, the extraneous substance sticking to the front surface thereof can be distinguished from what is sticking to the back surface before being detected.
2. Background Art
Flaw inspecting apparatus are generally used for detecting extraneous substances sticking to the surfaces of masking substrates (glass substrates) and silicon wafers for use in manufacturing semiconductor ICs, and to glass plates for use in liquid crystal panels and the like, the extraneous substances ordinarily including flaws such as defects of their surfaces themselves. Product quality is thus maintained at not lower than a certain level.
FIG. 5 shows a basic configuration of a flaw detecting optical system in an apparatus for detecting wafer surface flaws as a flaw inspecting apparatus of the sort described above by way of example. The optical system consists of a light projecting system 2 and a light receiving system 3. Laser beams emitted from a laser beam source 21 are focused by a light projecting lens 22. An optical spot is formed on the surface of a wafer plate 1 as an object under examination. When the optical spot scans the surface of the wafer plate 1 pursuant a revolving scanning or XY scanning method, it will be scattered at a place where a flaw is found. The scattered light is condensed by a condenser lens 31 in the light receiving system 3 and received by a light receiver 32. A flaw detection signal is thus obtained. A stopper 33 provided in the light receiving system 3 is inserted for improving the S/N ratio by cutting off the regular reflected light deriving from the laser beam.
With the basic configuration described above, various improvements have been introduced in light projecting and light receiving systems in not only optical systems for detecting wafer surface flaws but also those for detecting flaws on masking substrates, glass plates for use in liquid crystal panels and the like. For instance, it is commonly practiced to improve the condensation effect by means of optical fibers instead of the condenser lens 31 as a light receiving system.
A TFT-type liquid crystal panel is formed with an extra-fine liquid crystal pixel electrode and a thin-film transistor (TFT), formed by etching the surface of a glass plate. The surface of the glass plate (glass substrate) with the liquid crystal pixel electrode and TFT formed thereon is provisionally called a pixel formative plane, for instance. With extraneous substances sticking to the pixel formative plane, it is highly probable for a number of TFTs connected to each other to simultaneously malfunction. The extraneous substances sticking to that surface may seriously affect the quality of the liquid crystal panel as a product; this makes it necessary to use such a flaw detecting optical system for detecting the presence or absence of extraneous substances on the pixel formative plane of the glass substrate.
Incidentally, the glass substrate having a pixel formative plane is a plate which is translucent, as thin as about 1 mm and has high transmittance. Consequently, the problem is that extraneous substances sticking to the pixel formative plane (hereinafter simply called `surface extraneous substance(s)`) and those sticking to the back surface of the glass substrate (hereinafter simply called `back surface extraneous substance(s)`) may simultaneously be detected. The back surface of the glass substrate is normally used as a side where an image to be displayed is observed or where back light is transmitted therethrough when the glass substrate is assembled as part of a liquid crystal panel. This means the back surface of the glass substrate is only need to be a glass surface, and so long as extraneous substances sticking thereto are minute, they pose no problems; in other words, the extraneous substances are not regarded as flaws in many cases. In case these extraneous substances are detected as flaws, the otherwise good parts are assumed to be bad and the yield of parts tends to decrease, thus resulting in a heavy loss.
While the operation of detecting flaws on a glass substrate for a liquid crystal display panel is performed, only extraneous substances on a pixel formative plane on the surface side thereof as viewed from the liquid crystal are detected so that those on the back surface side may be ignored. However, the fact that the glass substrate is a thin plate having high transmittance makes it actually difficult to distinguish extraneous substances on the surface from those on the back surface.
The present inventors proposed the art of detecting extraneous substances on both sides separately, and the present applicants filed an application for a patent under the title "A Method of Detecting Surface Flaws on Liquid Crystal Panel", Japanese Patent Application No. 327966/1989, dated Dec. 18, 1989. Referring to FIGS. 6(a) and 6(b), a general description will subsequently be given of the method.
As shown in FIG. 6(a), a light projecting system 2 and receiving system 3 are provided above the pixel formative plane (hereinafter called the `surface`) of a glass substrate 1 as an object under examination, whereas a light projecting system 2 and receiving systems 3' are provided symmetrically about the glass substrate 1 on the back surface side thereof. Laser beams T, T' of substantially the same intensity are used respectively to form optical spots on the surface and the back surface, whereby alternate scanning is implemented. On the assumption that the sensitivities of the two light receiving systems 3, 3' are substantially identical, the detected signals of scattered light are compared. Of the detected signals in the light receiving system 3 on the surface side after the comparison is made, what is greater than the detected signal in the light receiving system 3' on the back surface side is defined as a flaw detecting signal concerning an extraneous substance, including a flaw, sticking to the surface.
A description will subsequently be given of the basic principle of making the above decision. FIG. 3(b) refers to cases where an extraneous substance Ps is sticking to the surface of the glass substrate 1 and where an extraneous substance Pb is sticking to the back surface thereof. In these cases, the light receiving system 3 on the surface side directly receives scattered light Rs deriving from the laser beam T at the surface extraneous substance Ps, and simultaneously irregular reflected light at the extraneous substance Pb on the back surface side. In other words, scattered light Rb at the extraneous substance Pb on the back surface side passes through the glass substrate 1 and reaches the light receiving system 3. Due to total reflection, the scattered light Rb attenuates as it passes through the glass substrate 1. As a result, the extraneous substance Ps on the surface side imparts to the light receiving system 3 scattered light that is more intense than what is directed to the light receiving system 3' on the back surface.
Such is also the case for an extraneous substance on the back surface side. The light receiving system 3' on the back surface side directly receives scattered light Rb' deriving from the laser beam T' at the extraneous substance Pb on the back surface side, and simultaneously irregular reflected light at the extraneous substance Ps on the surface side. In other words, scattered light Rs' at the surface extraneous substance Ps passes through the glass substrate 1 and reaches the light receiving system 3'. The scattered light Rs' also attenuates as it passes through the glass substrate 1. The extraneous substance Pb on the back surface side imparts to the light receiving system 3' scattered light more intense than what is directed to the light receiving system 3 on the surface side.
A comparison of the detected signals on both sides to find which one of the light receiving systems 3, 3', is receiving light that is more intense than the other makes it possible to determine whether the extraneous substance in question is located on the surface or the back surface side.
However, it has also frequently occurred that an extraneous substance on the surface side still remains inseparable from what exists on the back surface side when the above decision-making principle is actually applied and this has posed a problem. Since any apparatus using such a decision-making principle requires installing both light projecting and receiving systems on respective sides of an object under examination, the apparatus has proved unfavorable in view of its maintenance and the like as it tends to increase in size.